Pistons have crowns that are exposed to very high temperatures and pressures produced during combustion. Piston crowns are supported by piston bodies, which have relatively more material than the piston crowns. A cylindrical skirt is either integral with, or articulated to, the piston body. The cyclic nature of combustion and the general design of pistons results in very high thermal stresses in the piston crowns. To reduce the effects of thermal stress on piston crowns, it is known to provide a cooling system. Some piston cooling systems allow generally open exposure of an underside portion of the piston crown to cooling oil that splashes upward as the piston reciprocates within a chamber.
Other known piston cooling systems have generally closed, annular cooling chambers located adjacent the piston crown and have pressurized cooling fluid, typically oil, introduced into the chamber through an inlet port communicating with an oil jet located in an engine cylinder. Thereafter, the oil is re-circulated by exiting the closed chamber through an outlet and returning to an oil reservoir in the cylinder. One known piston having a closed cooling chamber incorporates a boss that is integral with the skirt sidewall. A bore drilled in the boss has an upper end defining an inlet port of the cooling chamber and a lower end of the bore is exposed to an oil jet for introducing oil into the cooling system.
Another known cooling design provides an inlet passage passing up through a connecting rod, radially through a piston pin, around a bearing surface recess and up through passages in a support member leading to a cooling chamber. However, such a cooling design is very complex and circuitous, requiring passages or bores in almost every component which results in increased manufacturing costs.
Another known location for a piston cooling passage is vertically through a pin boss leading up to a cooling chamber. However, pistons having cooling passages in a pin boss must have sufficient cross-sectional thickness to allow drilling a continuous bore vertically through the pin boss. Pin bosses having a smaller cross-sectional thickness or an irregular cross-section have not been provided with cooling passages because drilling a bore would break through an outer surface of the pin boss, resulting in severe leakage and an unusable passage. In addition, drilling a bore in a reduced cross-sectional thickness pin boss further weakens the pin boss, increasing stress loads and decreasing piston life.
Further, engine manufacturers continually seek to decrease the weight of their engines, including reducing the weight of component parts, such as pistons. At the same time, engine designers are unable to reposition the cooling nozzle jets because of space constraints. Therefore, pistons must be designed that are lighter in weight yet still have a main feature, such as a cooling passage, in generally the same location.